Method of manufacturing weldable, tough and high strength steel for structure members usable in the ashot-state and steel so made



3,328,211 Patented June 27, 1967 ice 3,328,211 METHGD F MANUFACTURENGWELDAEBLE, TUUGH AND HlGH STRENGTH STEEL FGR STRUCTURE MEMBERS USABLE INTHE AS- HQT-STATE AND d'iEEL $0 MADE Haiime Nahamura, Megurou, Tokyo-to,Japan, assignor to lshikawajimwfi'arima .bulrogyo Kahushild Keisha,Tokyo-to, Japan, a company ct lapan No Drawing. Filed Nov. 18, 1904,Ser. No. 412,231 Claims riority, application .lapan, Dec. 5, 1053, 38/6,401 Claims. (Cl. 140-32) This invention relates to a weldable, toughand high strength steel for structure members usable in the as-hotworkedstate and method of manufacture therefor.

There are a number of steels available today on commercial market thatpurport to have a tensile strength of 60 kg./mm. class together withductility, toughness and weldability that are adequate for fabricationof large steel structures such as ships, buildings, pressure vessels orbridges. Here, a term 60 kg./mrn. class is used to designate a range fortensile strength which is defined as between 58 kg/mm. and 70 kg./mm.both limits inclusive, in accordance with Japanese standards.

However, I am, as yet, unaware of one that is capable of developing thedesired set of mechanical properties, namely high strengths, both yieldand tensile, coupled with good ductility and/or toughness withoutsacrificing one in favor of the other, if left in the as-worked, forexample, as-rolled state. Therefore, heat treatment of one variety oranother has generally to be applied to those steel stocks, despiteconsiderable increase in price, to make the strength, ductility,toughness and weldability all adequate for a structure member steel.

Thus, an object of this invention resides in the provision ofinexpensive, weldable, tough and high strength steel for structuremembers that is perfectly usable in its as-hot-worked state, namelywithout any subsequent or independent heat treatment, having a set ofmechanical properties which is characterized by strength, ductility andtoughness all maintained at a high level. Another object is to determinethe proper composition for the steel described above so that the steelmay possess the set of mechanical properties as desired in theas-hotworked state when manufactured properly. The third is to give theproper method of hot-working for the particular steel described above,by which combination, namely proper hot-working on steel of propercomposition, it is only possible to obtain the set of mechanicalproperties desired. Other objects of this invention will in part bementioned in due course, and yet others will either be self-explanatoryor be readily understood or appreciated by those who are skilled in theart.

More specifically, this invention relates to a weldable, tough and highstrength steel for structure member that is capable of developing atensile strength of at least 58 kg./mm. but not over 70 kg./mm. yieldratio, the ratio of yield strength to tensile strength, of at least 72%,a ductility of at least 25% in elongation on gauge length of four timesthe specimen diameter at room temperature, and a notch impact toughnessof at least 80 ft.-1b. or 13.8 kg.-m./cm. at 0 C. by 2 mm. V notchCharpy test piece.

It also relates to a steel composition in the final product analysiswhich comprises by weight percent: carbon 0.08- 0.30%, silicon0.02-0.60%, manganese 0.52.50% nitrogen less than 0.045%, the whole orat least 0.010% of which is to be bound to aluminum in the form ofprecipitated aluminum nitride of 0.03-0.12%, the rest to any one orcombination selected from the group composed of: columbium less than0.20%, titanium less than 0.12% and zirconium less than 0.20%: freeuncombined nitrogen less than 0.004%, free metallic aluminum dissolvedin matrix as solid solution (solid solution metallic aluminum) less than0.15%, as well as at least one complementing alloying element selectedfrom the group consisting of: nickel 0.101.0%, chromium 0.10l.0%molybdenum 0.051.0%, copper ODS-1.0%, vanadium ODDS-0.30%, boron0.00050.01%; the balance substantially all iron with incidental orunavoidable impurities.

This invention relates further to a method of manu tacture for weldable,tough and high strength steel or structure member characterized by theset of mechanical properties defined earlier in the as-hot-worked state,the method comprising hot-working the steel in any manner in thetemperature range of 1200700 C., starting at. a temperature not above1200 C. and finishing at not below 700 C., giving the steel a reductionin thickness, calculated on the final thickness of the product, of atleast 20% in the temperature range below 1000 C. in one or more passes,thereafter cooling the steel.

As mentioned earlier briefly, in order to obtain a set of mechanicalproperties which is as good as or even better in the as-hot-worked statethan that may be realized after heat treatment, not only the hot-workingprocedure but also the composition of the steel stock must be carefullycontrolled. Namely, this invention is based on the discovery of theconditions governing 'both those ends. In the steel composition: Carbon,the foremost alloying element for steel, is necessary by at least 0.08%to ensure the 60 kg./mm. class tensile strength when co-prescribed withother ingredients, but tends to spoil the weldability of the steel whenpresent in excess of 0.30%, the preferred range falling in from about0.10% to 0.25%.

Silicon is added as deoxidizing agent for which at least 0.02% isneeded, but tends to excessively harden the matrix by solid solutioneffect and to affect unfavorably the ductility when present by more than0.60%, the preferable range being from about 0.3 to 0.5%.

Manganese, which is also an active deoxidizer, is an element that iscapable of hardening the steel without impairing the weldability, forwhich purpose at least 0.5% is needed. However, when present in excessof 2.5%, it tends to stabilize the lower baim'te in the as-hot-workedsteel to give rise to a higher strength at a considerable expense in theductility and toughness. Thereupon, the manganese content is limited tofrom 0.5 to 2.5%, of which 0.5 to 2.0% is the preferred range.

Aluminum nitride component of at least 0.03% on nitrogen of about 0.01%is necessary to ensure a high ductility and toughness. However, we haveobserved in attempting to provide precipitated aluminum nitride morethan about 0.12%, or, in terms of nitrogen bound to aluminum, 0.041%,the nonmetalli-c inclusions, particularly oxides of aluminum and siliconthat are inevitably retained in the steel, increases rapidly and thecieanness, and with it the ductility and toughness, of the steelcorrespondingly declined. The favorable effect the precipitated aluminumnit-ride exerts on the toughness and ductility itself, on the otherhand, tends to saturate as the content approaches this 0.12% level.

Some solid solution metallic aluminum content is unavoidable insofar asmore aluminum must be provided over what is needed for thestoichiometric combination with available nitrogen to ensure the desiredaluminu r n nitride content, but it is an unwelcome constituent for thesteel because aluminum dissolved in solid solution with the matrix tendsto coarsen the grain and thence to affect adversely the mechanicalproperties, particularly the low temperature toughness, of the steel. Wehave found that it is allowable only to 0.15% in the steels of thisinvention.

Of other metallic nitrides than that of aluminum, we found those ofcolumbium, titanium and zirconium are effective for this invention whenexisting in conjunction with aluminum nitride without causing the oxideor grain coarsening trouble of the former. For this purpose, columbiumless than 0.20%, titanium less than 0.12%, zirconium less than 0.20% maybe provided. However, when present in excess, they tend to form oxideand carbide which would act as the crack formation site, and thetoughness of the steel is adversely aifected. Free uncombined nitrogenis again unavoidable and on the whole unwelcome, the less of it beingthe better, for its unfavorable etfect on the low temperature toughnesscompletely overshadows its favorable effect on the yield point. However,with an aluminum nitride content of more than 0.03%, free uncombinednitrogen of up to 0.004% may be allowed for steel of this invention. Theprovision of columbium, titanium and zirconium helps to reduce the freeuncombined nitrogen further, and particularly with the last two, thecontent of the latter can be reduced. to the order of or less than0.001%.

The above-listed elements, particularly carbon, silicon, manganese, andaluminium nitride, are the fundamental constituents of steel accordingto the invention. However, a satisfactory combination of high strengthand good toughness cannot always be guaranteed bycarbon-siliconmanganese-aluminium nitride combination alone if the size,e.g. the thickness of the product becomes large, say more than one-halfinch.

Here resides the significance of other alloying element or elements thatcomplement this shortcoming of the fundamental composition. Namely, wehave found that any one or combination of the following elements isfurther necessary for the purpose of this invention already set forth.

Nickel is useful to increase the strength and particularly the toughnessof the steel as well as to counter the hot shortness caused by thecopper component, when present, and indeed the more of it, the better.However, considering the price with regard to the gain in mechanicalproperties, a content between 0.10 to 1.0%, preferably about 0.2 to 0.8%was found to be proper.

Chromium is advantageous to ensure a high strength in the as-hot-workedstate, though it tends to adversely affect the toughness, hence a rangeof 0.10 to 1.0%, preferably 0.3 to 0.8%, is useful.

Molybdenum, like chromium, helps to strengthen the steel but also tendsto impair the toughness, hence a range of 0.05 to 1.0%, preferably 0.1to 0.6%, should be present.

Copper is effective to improve the corrosion resistivity as well as toraise the strength level of the steel, but tends to develop hotshortness and to affect unfavorably on the weldability and lowtemperature toughness, hence we use a range of 0.05 to 1.0%, preferably0.1 to 0.8%.

Vanadium also strengthens the steel when dissolved. in the matrix assolid solution, but tends to enhance the weld-cracking trouble.Balancing those against its price, a range of 0.005 to 0.30%, preferably0.02 to 0.1%, was found to be satisfactory.

Boron imparts additional strength without impairing the toughness orweldability in a range of 0.0005 to 0.01%, preferably 0.0005 to 0.005%.

We observed that even those steels composed carefully according toabove-described principles did not come up with the expected set ofmechanical properties defined earlier if hot-worked in a commonlyemployed manner, for example, rolled in the range of 1300 to "900 C.,starting in the vicinity of the former and finishing close to thelatter, the overall reduction in thickness 'being distributed more orless evenly between passes. We have traced this failure to theprecipitation characteristic of the metallic nitride component,particularly aluminum nitride, and to the unsatisfactory state ofdispersion thereof. After many tests and experiments, we have found thatnot only the hot-working temperature range but also the working schedulemust be specified. Namely, in order for steels of this invention todevelop the set of mechanical properties specified, the temperaturerange in which to hot-work must be 1200 to 700 C., that is to say, thestarting temperature not above 1200 C. and finishing temperature notbelow 700 C., and the hotworking pass schedule such that a reduction inthickness, calculated on the final thickness of the product, of at least20% be given in one or more passes which are to be conducted at atemperature below 1000 C. Thereafter the steel thus prepared maybecooled in any manner, for example, it may be left in air.

Although the exact reason for this difference has not been explained, Ibelieve that the nitride particles, which have already precipitatedduring the prior stages of steelmaking, comprising the ingot making andblooming, play an important role. Therefore, the soaking temperature,i.e. the starting temperature for hot-working must not be so high as tocause the dissolution of nitride precipitates; a temperature range of1100 to 1200 C. has been found proper with regard to the preservation ofonce precipitated nitride particles and to the ease of hot-working aswell.

The details and merits of this invention will further be apparent fromthe example described below in which a number of steels of thisinvention or those acquired on the commercial market were hot-worked byrolling as indicated.

Example to this invention method, and Steel 11 according to commonpractice, while Table 3 summarizes the mechanical properties of productsteels.

TABLE 1.CHEMICAL COMPOSITION BY PRODUCT ANALYSIS (WT. PERCENT) Steel CSi Mn Ni Cl Mo Cu V Cb Ti Zr 13 AlN 0. 18 0. 40 1. 43 O. 052 0. 16 O.40 1. 44 0. 032 0. 19 0. 4O 1. 50 0. 055 0. 20 0. 40 1. 51 O. 052 0. 180. 40 1. 28 0. 036 0. 17 0. 4O 1. 23 0. 049 0. 18 0. 40 l. 46 0. 055 0.19 0. 41 1. 50 0. 052 0. 18 0. 19 0. 53 0. 047 0. 17 0. 26 0. 52 0. 0030. 18 0. 56 1. 48 0. 005

TABLE 111 Al in Nin Nin No. AlN Met. Al Nitride Former Other than Al.AlN Tot. N F. N AlN Other Nitride 0.052 0.051 0.035 0.021 0.004 0.0170.032 0.033 0.021 0.014 0.003 0.011 0.055 0. 003 ob, 0.019 0.037 0. 0210.001 0. 013 0.002 0.052 0.010 Zr, 0.03s 0.035 0.024 0. 002 0.017 0. 0050.035 0.023 Zr,0047;B 0001- 0.024 0.017 0.001 0.012 0.004 0. 049 0.034Zr, 0.033 0. 021 0.003 0.016 0.002 0.055 0.019 v, 0.04; Ti, 0. 0.0370.022 0.002 0.013 0.002 0.052 0. 012 v, 0.04; Cb, 0.04 0.035 0.024 0.002 0.017 0.005 0.047 0. 013 Cb, 0.041; Zr, 0.093; B,0.001 0.032 0.0230.000 0.015 0.008 0.003 0.031 0. 002 0. 003 0.007 0. 001 11 0.005 0.0000.003 0.007 0.005 0.002

TABLE ROLLING DATA Although only the fiat-rolling process was used indescribing the method of the invention, we hold no special SmkingRolling Schedule preference for it in the hot-working method. Forging,Steel Tefilpem swaging, section-rolling or any other known processes ofture, 0. Starting Finishing Finishing h t. r

Tempew Tempem Reduction work ng a e equally sat1stactory nasmuch as theture, 0. ture, 0. Per ent workrng is carried out in accordance w1th themethod of this invention. 1,150 1,100 850 40 Having described heretoforethe principles and applica- 22g t10ns of this invention, I do not wishto be confined to the 1:160 1:100 840 50 factual examples shown, butonly by the claims as set 1,150 1,110 840 45 fo th 1,150 1, 090 800 251,150 1, 000 320 34 I 1.140 1,100 49 1. A method of manufacturingweldable, tough and 1, 100 1, 070 190 44 1,140 1,100 800 41 highstrength steel for structure members usable in the 1.320 1,270 950 10 n0as-hot-worked state, said steel consisting of carbon 0.08- 0.30%,silicon 0.02-0.60%, manganese 0.5-2.5 nitro- 1 Reduction in thicknesscalculated on product, obtained at temperatures below 1,000 C.

TABLE 3.MECHANICAL PROPERTIES OF THICK PLATE Yield Tensile ElonvEn,Yield Steel Strength, Strength, gation, kg.-m/cm. Ratio,

lrglmm. kg./1nm. Percent Percent 1 Gauge length mm., 10 mm. diameterspecimen. 2 Charpy 2 mm. V notch specimen, at 0 C.

It will be seen in those tables that the steels with chemicalcomposition according to this invention and prepared in accordance withthe method of the invention all exhibit the set of mechanical propertiesspecified. It is further noted that Steel 10, which was preparedaccording to the method of the invention from commercial Cr-Mo typesteel stock, compares very favorably with Steel 11, which is in fact ahigh quality low-manganese grade commercial steel. However, the lowtemperature notch toughness of Steel 10 fails to attain the 80 ft.-lb.(13.8 kg.-m/cm. level, or the yield ratio the 72% requirement, becausethe aluminum nitride content is not as much as required in thisinvention. Those results shown by Steels 1 to 9 of this invention aresimply compared with those of Steels 10 and 11 for the superiority to beappreciated.

It will further be seen that a wide variety of combina- :tions ofmechanical properties fit for particular service requirements can begained by properly selecting the chemical composition and/ormanufacturing procedures. For example, Steels 1, 3 and 8 mayadvantageously be used for structures designed on the yield stress basisfor their high yield points; here the presence of columbium, partly inthe form of columbium nitride existing along with aluminum nitride, isappreciated, while the application of Steels 3, 7, 8 and 9 may be quiteprofitable in the low temperature service structures for their highnotch toughness.

gen less than 0.045%, at least 0.010% of which is to be bound toaluminum in the form of precipitated aluminum nitride of 0.03-0.12%, therest to any one selected from the group composed of: columbium less than0.20%, titanium less than 0.12% and zirconium less than 0.20%, and acombination thereof; free uncombined nitrogen less than 0.004%, freemetallic aluminum dissolved in the matrix as solid solution less than0.15 as well as at least one complementing alloying element selectedfrom the group consisting of: nickel 0.10-1.0%, chromium 0.10-1.0%,molybdenum ODS-1.0%, copper ODS-1.0%, vanadium 0.005-0.30%, boron0.0005-0.01%; the balance substantially all iron with incidental andunavoidable impurities, the method consisting of heating the steel to asoaking temperature in the range of 1100-1200 C., hot-Working the steelin the temperature range of 1200 700 C., starting the hot working at atemperature not above 1200" C. and finishing at a temperature not below700 C., giving the steel a reduction in thickness, calculated on thefinal thickness of the product, of at least 20% in at least one pass inthe temperature range below 1000" C., and thereafter cooling the steel.

2. The method according to claim 1, wherein the steel consists of carbon0.10-0.25%, silicon 0.3-0.5%, manganese 0.5-2.0%, precipitated aluminumnitride 0.03- 0.l2%, metallic aluminum dissolved in the matrix as solidsolution less than 0.15%, at least one from: the group consisting ofcolumbium less than 0.20%, titanium less than 0.12%, zirconium less than0.20%, free uncombined nitrogen less than 0.004%, as well as at leastone from: the group consisting of nickel 0.2-0.8%, chromium 03-08%,molybdenum 0.1-0.6%, copper 0.1-0.8%, vanadium 0.02O.1%, boron'0.0005-0.005%; the balance substantially all iron with incidental andunavoidable impurities.

3. A weldable, tough and high strength steel for structure membersusable in the as-hot-Worked state having a tensile strength of at least58 kg./mm. but not over 7 kg./rnm. a yield ratio of at least 72%, aductility at room temperature of at least 25% as measured on guagelength of four times the specimen diameter, and a notch impact toughnessof at least ft.-lb. or 13.8 kg.-m./cm. at 0 C. as measured by 2 mm. Vnotch Cha-rpy test piece, said steel consisting of the ingredients andhaving been made by the steps defined in claim 1.

7 8 4. A weldable, tough and high strength steel for struc- 3,155,496l1/ 1964 Nakamura 75-124 ture members usable in the as-hot-worked statehaving 3,155,549 11/1964 Nakamura 75l24 X a tensile strength of at least58 kg./mrn. but not over 70 3,173,782 3/1965 Melloy t v 1 7 5 123kg./mm. a yield ratio of at least 72%, a ductility at 3 259 3 7 19Nakamura 75.424 room temperature of at least 25% as measured on guage 5length of four times the specimen diameter, and a notch FOREIGN PATENTSimpact tou hness of at least 80 ft.-1b. or 13.8 kg.-m./cm. at 0 C. a:measured by 2 mm. V notch Charpy test 786993 11/1957 Great i i piece,said steel consisting of the ingredients and having 8081556 2/1959 GreatBmam' been made by the steps defined in claim 2. 10 830,669 3 1960 Greati i 904,886 9/1962 Great Bntam. References Cited UNITED STATES PATENTSDAVID L. RECK, Primary Examiner.

3,010,822 11/1961 Altenburger et a1 75-423 3,155,495 1l/1964 Nakamura75124 15 SAITO, Assistant Exammer,

1. A METHOD OF MANUFACTURING WELDABLE, TOUGH AND HIGH STRENGTH STEEL FORSTRUCTURE MEMBERS USABLE IN THE AS-HOT-WORKED STATE, SAID STEELCONSISTING OF CARBON 0.080.30% SILICON 0.02-0.60%M MANGANESE 0.5-2.5%,NITROGEN LESS THAN 0.345%, AT LEAST 0.010% OF WHICH IS TO BE BOUND TOALUMINUM IN THE FORM OF PRECIPITATED ALUMINUM NITRIDE OF 0.03-0.12%, THEREST TO ANY ONE SELECTED FROM THE GROUP COMPOSED OF: COLUMBIUM LESS THAN0.20%, TITANIUM LESS THAN 0.12% AND ZIRCONIUM LESS THAN 0.20%, AND ACOMBINATION THEREOF; FREE UNCOMBINED NITROGEN LESS THAN 0.004%, FREEMETALLIC ALUMINUM DISSOVED IN THE MATRIX AS SOLID SOLUTION LESS THAN0.15%, AS WELL AS AT LEAST ONE COMPLEMENTING ALLOYING ELEMENT SELECTEDFROM THE GROUP CONSISTING OF: NICKEL 0.10-1.0%, CHROMIUM 0.10-1.0%,MOLYBDENUM 0.05-1.0%, COPPER 0.05-1.0%, VANADIUM 0.005-0.30%, BORON0.0005-0.01%; THE BALANCE SUBSTANTIALLY ALL IRON WITH INCIDENTAL ANDUNAVOIDABLE IMPURITES, THE METHOD CONSISTING OF HEATING THE STEEL TO ASOAKING TEMPERATURE IN THE RANGE OF 1100*- 1200*C., HOT-WORKSING THESTEEL IN THE TEMPERATURE RANGE OF 1200*700*C., STARTING THE HOTWORKINGAT A TEMPERATURE NOT ABOVE 1200*C. AND FINISHING AT A TEMPERATURE NOTBELOW 700*C., GIVING THE STEEL A REDUCTION IN THICKNESS, CALCULATED ONTHE FINAL THICKNESS OF THE PRODUCT, OF AT LEAST 20% IN AT LEAST ONE PASSIN THE TEMPERATURE RANGE BELOW 1000*C., AND THEREAFTER COOLING THESTEEL.